The invention relates to a polyethylene composition and objects made therefrom. The invention also relates to a process for the manufacture of foamed objects from the polyethylene composition.
Foamed objects from low density polyethylene (LDPE) can be made by techniques which have been known for a long time. (Plastic Foams Part 1, Kurt C. Frisch & James H. Saunders (Eds.), pp 281-292). Such a polyethylene has a density of between 915 and 940 kg/m.sup.3 and is made in a high pressure process with the aid of one or more radical initiators. Foamed products from this LDPE have excellent properties that can be adjusted at will to suit any of a broad range of applications, for example by making the cells open or closed as desired, or large or small, in a wide variety of foam densities and foam shapes.
Thanks to these properties, objects from foamed LDPE are broadly applicable, e.g. as insulation material. Open-cell foams, for example, are used for acoustic insulation and closed-cell foams for thermal insulation. Further, LDPE foams are suitable for application as packaging for fragile or delicate objects, on account of their good energy-absorbing properties and their generally high resistance to chemicals.
The several applications impose different requirements on the foam, e.g. softness, flexibility, cold brittleness, environmental stress crack resistance (ESCR) and the like. It is known that these properties are increasingly present if the foams are made of LDPE with lower densities and/or with increasing amounts of incorporated polar comonomers, e.g. vinyl acetate, acrylate, methacrylate, methyl methacrylate and the like. When such polar copolymers are used, the above-mentioned properties of LDPE foams can to a greater extent be adjusted to the requirements than in the case of the homopolymer LDPE.
However, a disadvantage of polar copolymer foams is that, although the flexibility increases with the amount of comonomer incorporated, the high temperature resistance of the foam decreases. The softening and melting range of polar copolymers lies at lower temperatures than the softening and melting range of LDPE homopolymer. This limits the field of application of flexible foams. Further, the polar copolymers are more likely to give rise to sticking problems during their conversion to (foamed) objects.
The object of the invention is to obtain polyethylene compositions which, when processed to foamed objects, have a high resistance to high temperatures as well as a high flexibility.
This object is achieved by a polyethylene composition comprising 20-98 wt% branched polyethylene (a) with a density of between 915 and 940 kg/m.sup.3 and a melt flow index of between 0.05 and 40 dg/minute, prepared by a high-pressure radical process, and 2-80 wt% of a substantially linear polyethylene (b) with a density of between 850 and 915 kg/m.sup.3, a melt flow index of between 0.05 and 25 dg/minute and a DSC crystallinity at 23.degree. C. of at least 10%, made with the aid of a transition metal catalyst, the difference between the highest DSC crystallization temperature of branched polyethylene (a) and the highest DSC crystallization temperature of linear polyethylene (b) being at most 10.degree. C. and the mixture having a modulus of elasticity of at most 280 N/mm.sup.2.